Donald Trump last year emerged from his first summit with Kim Jong Un—a historic meeting, to be sure—and proudly declared that “there is no longer a Nuclear Threat from North Korea.” He’d spent the Singapore summit glad-handing the North Korean dictator, and had come away with only the vaguest of agreements to denuclearize the peninsula, but for the president, that was enough: Kim had stopped playing with his rockets in public, and he’d made a new best bud. Nobel Peace Prize, please.

But for all his bluster, the June meeting brought the two countries no closer to a concrete accord—a fact that became painfully obvious Thursday as the president’s second round of talks in Hanoi collapsed over disagreement on just the first steps toward denuclearization. Kim, evidently, had wanted the United States to drop all its sanctions, just to end one part of its nuclear program—an enormous concession for the U.S. that would effectively allow Pyongyang to continue other elements of its nuclear development. That brought an abrupt end to the discussions, with a lunch between the pair canceled and both sides leaving the negotiating table early, possibly even further from a deal than when they started.

“Sometimes you have to walk,” Trump told reporters.

It was a significant setback for the president, who has touted the fragile progress on the peninsula as his main foreign-policy achievement. But it also represents a potentially dangerous return to the tensions that dominated 2017, when Trump and Kim spent the better part of a year lobbing schoolyard taunts and nuclear threats at one another. “I worry about the consequences,” Jean H. Lee, a Korea expert at the Wilson Center, toldThe New York Times. “Did these two leaders and their teams build up enough goodwill to keep the lines of communication open, or are we headed into another period of stalled negotiations—or worse, tensions—that would give the North Koreans more time and incentive to keep building their weapons program?”

Right now, it’s not clear, though both sides have attempted to gloss over the failed talks. Kim continued to suggest he was willing to denuclearize, and told Trump he would maintain the freeze on nuclear testing. The president, meanwhile, continued to praise the North Korean dictator. “This was very friendly,” Trump said of his walkout. “We shook hands.” Indeed, Trump’s flattery of the noted human-rights abuser raised eyebrows—particularly when he said he would “take [Kim] at his word” when the dictator denied knowledge of the maltreatment of American student Otto Warmbier, who died upon being returned to the U.S. after spending more than a year in a North Korean prison.

Indeed, Trump has bent over backward to accommodate Kim. In entering talks, his administration reportedly dropped one of its key demands—a full accounting of all of North Korea’s nuclear weapons and ballistic-missile programs—in what appeared to be a significant lowering of expectations for Pyongyang. That the talks still blew up before they could begin in earnest represents a major failure for the president. Trump has never been the master negotiator he’s played on TV, but for years, the caricature has stuck. In addressing one of the most challenging foreign-policy problems of his presidency, however, it’s clear that optics won’t be enough.